This invention relates to horizontal-axis turbines. It has particular application to large wind turbines, although it is also useful with such turbines driven by other fluids, such as water. Such turbines include a number of blades, most commonly three, mounted to a horizontal shaft. For convenience, such turbines are referred to herein as horizontal axis wind/water turbines or HAWTs, regardless of the operating fluid.
The efficiencies of HAWTs in the “stand alone” or wind farm configurations have been improving for decades. Particularly in the last thirty years, the efficiency of the wind energy extraction was attempted by                a) Increasing the number of blades. This never worked, the standard number now is three blades per rotor.        b) Improving the aerodynamic efficiencies of blade design: Blade design is now almost perfected.        c) Increasing the size of the blades (rotors): Blade size is now reaching the limits of practicality. The ever increasing blade sizes lead to great cost-efficiencies but present great structural problems. The cost-efficiencies are now off-set by the exponential cost increases of the blade attachments which would still allow the blade pitch dynamic adjustments. The forces which the pitch motors and gears have to overcome are enormous.        
The scaling up efforts of the last thirty years are therefore now reaching their limits.
The size of utility scale HAWTs has grown enormously with intent to improve the cost of Kilowatt/hour produced. Scaling up takes advantage of the obvious economy of scale. The 5 Megawatt HAWTs currently installed have a rotor diameter larger than a football field. Each blade often exceeds 70 meters in length. New ways, rather than scaling up, need to be found to improve the single HAWT's power output. The limits imposed by structural material characteristics are now being approached. The transportation and assembly cost of such giant parts will also limit further future scaling-up efforts.
Numerous examples of such HAWTs have been described in the patent literature, including Stiesdal, U.S. Pat. No. 7,956,484, Cairo, U.S. Pat. No. 7,993,103, and Wobben, U.S. Pat. No. 7,708,530, for example.
Previous attempts have also been made to increase HAWT performance by adding one or more rotors. Decades ago, attempts were made to add another rotor on the same shaft. There have been many attempts to use an additional rotor or rotors behind the front rotor of HAWT wind turbines. The original reasoning for such dual rotor HAWTs was sound, since the second rotor would share the same rotational shaft, and would thus help to drive the attached single generator. The “power density” of two rotors per single tower would increase the overall power output of the wind farm without doubling the cost or doubling the land area required. The other cost-effectiveness would have been derived from sharing the same costly single tower and single nacelle. Recent attempts along these lines have been made by Danny J. Smith as exemplified in U.S. Published Patent Application No. 2011/0223017.
These attempts, however, have met with limited success. While theoretically possible, the diminished performance of the second rotor placed in the “downwash” of the first front rotor never justified the additional rotor cost and additional structural and coupling mechanisms costs of adding such a secondary rotor.